Compositionally and structurally disordered multiphase nickel hydroxide positive electrode for alkaline rechargeable electrochemical cells

ABSTRACT

A positive electrode for use in alkaline rechargeable electrochemical cells comprising: a material comprising a compositionally and structurally disordered multiphase nickel hydroxide host matrix which includes at least one modifier. A process for forming a high loading uniformly distributed multiphase substantially nitrate free sintered positive electrode for use in an alkaline rechargeable electrochemical cell, the process comprising: (1) fabricating sintered electrode material by forming a slurry of nickel powder, water, carboxy methyl cellulose binder, methyl cellulose binder, and a poly(ethylene oxide) polymer; spreading the slurry on a preoxidized perforated nickel substrate; drying the slurry; and sintering the slurry; (2) impregnating the sintered electrode material using multiple impregnation cycles to attain high loading; and (3) forming the impregnated sinter into positive electrode material by presoaking the impregnated sinter in NaOH presoak tanks to substantially eliminate nitrates; brushing the presoaked impregnated sinter in a surface brushing station; charging the brushed impregnated sinter; discharging the charged impregnated sinter; rinsing the discharged impregnated sinter; and drying the rinsed impregnated sinter to complete the formation of positive electrode material.

This is a continuation of application Ser. No. 07/975,031 filed Nov. 12,1992, now U.S. Pat. No. 5,344,728.

FIELD OF THE INVENTION

The present invention relates generally to an optimized nickel hydroxidepositive electrode. More specifically, this invention relates tooptimized nickel hydroxide positive electrodes for rechargeable alkalinecells.

BACKGROUND OF THE INVENTION

In rechargeable alkaline cells, weight and portability are importantconsiderations. It is also advantageous for rechargeable alkaline cellsto have long operating lives without the necessity of periodicmaintenance. Rechargeable alkaline cells may be used as directreplacements for primary AA, C, and D cells in numerous consumer devicessuch as calculators, portable radios, and cellular phones. They areoften configured into a sealed power pack that is designed as anintegral part of a specific device. Rechargeable alkaline cells can alsobe configured as larger cells that can be used, for example, inindustrial, aerospace, and electric vehicle applications.

The best rechargeable alkaline cells are ones that can operate as an"install and forget" power source. With the exception of periodiccharging, a rechargeable alkaline cell should perform without attentionand should not become a limiting factor in the life of the device itpowers.

There are two basic types of rechargeable alkaline cells: nickel cadmium("NiCd") cells and nickel metal hydride ("Ni--MH") cells.

In a NiCd cell, cadmium metal is the active material in the negativeelectrode. NiCd cells use a positive electrode of nickel hydroxidematerial. The negative and positive electrodes are spaced apart in thealkaline electrolyte.

Upon application of an electrical potential across the materials of aNiCd cell, the negative electrode undergoes to the following reaction:##STR1## During discharge, this reaction is reversed, Cd is oxidized toCd(OH)₂ and electrons are released. The reactions that take place at thepositive electrode of a Ni--Cd cell are also reversible. For example,the reactions at a nickel hydroxide positive electrode in a nickelcadmium cell are: ##STR2##

In general, Ni--MH cells utilize a negative electrode that is capable ofthe reversible electrochemical storage of hydrogen. Ni--MH cells usuallyemploy a positive electrode of nickel hydroxide material. The negativeand positive electrodes are spaced apart in the alkaline electrolyte.

Upon application of an electrical potential across a Ni--MH cell, theNi--MH material of the negative electrode is charged by theelectrochemical absorption of hydrogen and the electrochemicalgeneration of hydroxyl ions: ##STR3## The negative electrode reactionsare reversible. Upon discharge, the stored hydrogen is released to forma water molecule and evolve an electron.

The reactions that take place at the nickel hydroxide positive electrodeof a Ni--MH cell are: ##STR4## This is the identical reaction thatoccurs in a NiCd cell.

Ni--MH cells can be further classified as V--Ti--Zr--Ni (Ovonic or AB₂)based or AB₅ (mischmetal) alloys depending on the type of hydrogenstorage material used as the negative electrode. Both types of materialare discussed in detail in copending U.S. patent application Ser. No.07/934,976, now U.S. Pat. No. 5,277,999 to Ovshinsky and Fetcenko, thecontents of which are incorporated by reference.

The first hydrogen storage alloys to be investigated as batteryelectrode materials were TiNi and LaNi₅. Many years were spent instudying these simple binary intermetallics because they were known tohave the proper hydrogen bond strength for use in electrochemicalapplications. Despite extensive efforts, however, researchers foundthese intermetallics to be extremely unstable and of marginalelectrochemical value due to a variety of deleterious effects such asslow discharge, oxidation, corrosion, poor kinetics, and poor catalysis.The initial use of these simple alloys for battery applications reflectthe traditional bias of battery developers toward the use of singleelement couples of crystalline materials such as NiCd, NaS, LiMS, ZnBr,NiFe, NiZn, and Pb-acid. In order to improve the electrochemicalproperties of the binary intermetallics while maintaining the hydrogenstorage efficiency, early workers began modifying TiNi and LaNi₅systems.

The modification of TiNi and LaNi₅ was initiated by Stanford R.Ovshinsky at Energy Conversion Devices (ECD) of Troy, Mich. Ovshinskyand his team at ECD found that reliance on simple, relatively purecompounds was a major shortcoming of the prior art. Prior work haddetermined that catalytic action depends on surface reactions at sitesof irregularities in the crystal structure. Relatively pure compoundswere found to have a relatively low density of hydrogen storage sites,and the type of sites available occurred accidently and were notdesigned into the bulk of the material. Thus, the efficiency of thestorage of hydrogen and the subsequent release of hydrogen to form waterwas determined to be substantially less than that which would bepossible if a greater number and variety of active sites were available.

Ovshinsky had previously found that the number of surface sites could besignificantly increased by making an amorphous film that resembled thesurface of the desired relatively pure materials. As Ovshinsky explainedin Principles and Applications of Amorphicity, Structural Change, andOptical Information Encoding, 42 Journal De Physique at C4-1096 (Octobre1981):

Amorphicity is a generic term referring to lack of X-ray diffractionevidence of long-range periodicity and is not a sufficient descriptionof a material. To understand amorphous materials, there are severalimportant factors to be considered: the type of chemical bonding, thenumber of bonds generated by the local order, that is its coordination,and the influence of the entire local environment, both chemical andgeometrical, upon the resulting varied configurations. Amorphicity isnot determined by random packing of atoms viewed as hard spheres nor isthe amorphous solid merely a host with atoms imbedded at random.Amorphous materials should be viewed as being composed of an interactivematrix whose electronic configurations are generated by free energyforces and they can be specifically defined by the chemical nature andcoordination of the constituent atoms. Utilizing multi-orbital elementsand various preparation techniques, one can outwit the normalrelaxations that reflect equilibrium conditions and, due to thethree-dimensional freedom of the amorphous state, make entirely newtypes of amorphous materials--chemically modified materials . . .

Once amorphicity was understood as a means of introducing surface sitesin a film, it was possible to produce "disorder" that takes into accountthe entire spectrum of local order effects such as porosity, topology,crystallites, characteristics of sites, and distances between sites.Thus, rather than searching for material modifications that would yieldordered materials having a maximum number of accidently occurringsurface irregularities, Ovshinky's team at ECD began constructing"disordered" materials where the desired irregularities were tailormade. See, U.S. Pat. No. 4,623,597, the disclosure of which isincorporated by reference.

The term "disordered," as used herein corresponds to the meaning of theterm as used in the literature, such as the following:

A disordered semiconductor can exist in several structural states. Thisstructural factor constitutes a new variable with which the physicalproperties of the [material]. . . can be controlled. Furthermore,structural disorder opens up the possibility to prepare in a metastablestate new compositions and mixtures that far exceed the limits ofthermodynamic equilibrium. Hence, we note the following as a furtherdistinguishing feature. In many disordered [materials]. . . it ispossible to control the short-range order parameter and thereby achievedrastic changes in the physical properties of these materials, includingforcing new coordination numbers for elements . . .

S. R. Ovshinsky, The Shape of Disorder, 32 Journal of Non-CrystallineSolids at 22 (1979) (emphasis added).

The "short-range order" of these disordered materials are furtherexplained by Ovshinsky in The Chemical Basis of Amorphicity: Structureand Function, 26:8-9 Rev. Roum. Phys. at 893-903 (1981):

[S]hort-range order is not conserved . . . Indeed, when crystallinesymmetry is destroyed, it becomes impossible to retain the sameshort-range order. The reason for this is that the short-range order iscontrolled by the force fields of the electron orbitals therefore theenvironment must be fundamentally different in corresponding crystallineand amorphous solids. In other words, it is the interaction of the localchemical bonds with their surrounding environment which determines theelectrical, chemical, and physical properties of the material, and thesecan never be the same in amorphous materials as they are in crystallinematerials . . . The orbital relationships that can exist inthree-dimensional space in amorphous but not crystalline materials arethe basis for new geometries, many of which are inherentlyanti-crystalline in nature. Distortion of bonds and displacement ofatoms can be an adequate reason to cause amorphicity in single componentmaterials. But to sufficiently understand the amorphicity, one mustunderstand the three-dimensional relationships inherent in the amorphousstate, for it is they which generate internal topology incompatible withthe translational symmetry of the crystalline lattice . . . What isimportant in the amorphous state is the fact that one can make aninfinity of materials that do not have any crystalline counterparts, andthat even the ones that do are similar primarily in chemicalcomposition. The spatial and energetic relationships of these atoms canbe entirely different in the amorphous and crystalline forms, eventhough their chemical elements can be the same . . .

Short-range, or local, order is elaborated on in U.S. Pat. No. 4,520,039to Ovshinsky, entitled Compositionally Varied Materials and Method forSynthesizing the Materials, the contents of which are incorporated byreference. This patent discusses how disordered materials do not requireany periodic local order and how, by using Ovshinsky's techniques,spatial and orientational placement of similar or dissimilar atoms orgroups of atoms is possible with such increased precision and control ofthe local configurations that it is possible to produce qualitativelynew phenomena. In addition, this patent discusses that the atoms usedneed not be restricted to "d band" or "f band" atoms, but can be anyatom in which the controlled aspects of the interaction with the localenvironment plays a significant role physically, electrically, orchemically so as to affect the physical properties and hence thefunctions of the materials. These techniques result in means ofsynthesizing new materials which are disordered in several differentsenses simultaneously.

By forming metal hydride alloys from such disordered materials,Ovshinsky and his team were able to greatly increase the reversiblehydrogen storage characteristics required for efficient and economicalbattery applications, and produce batteries having high density energystorage, efficient reversibility, high electrical efficiency, bulkhydrogen storage without structural change or poisoning, long cyclelife, and deep discharge capability.

The improved characteristics of these alloys result from tailoring thelocal chemical order and hence the local structural order by theincorporation of selected modifier elements into a host matrix.Disordered metal hydride alloys have a substantially increased densityof catalytically active sites and storage sites compared to conventionalordered materials. These additional sites are responsible for improvedefficiency of electrochemical charging/discharging and an increase inelectrical energy storage capacity. The nature and number of storagesites can even be designed independently of the catalytically activesites. More specifically, these alloys are tailored to allow storage ofhydrogen atoms at bonding strengths within the range of reversibilitysuitable for use in secondary battery applications.

Based on the pioneering principles described above, a family ofextremely efficient electrochemical hydrogen storage materials wereformulated. These are the Ti--V--Zr--Ni type active materials such asdisclosed in U.S. Pat. No. 4,551,400 ("the '400 Patent") to Sapru, Hong,Fetcenko, and Venkatesan, the disclosure of which are incorporated byreference. These materials reversibly form hydrides in order to storehydrogen. All the materials used in the '400 Patent utilize a genericTi--V--Ni composition, where at least Ti, V, and Ni are present with atleast one or more of Cr, Zr, and Al. The materials of the '400 Patentare generally multiphase materials, which may contain, but are notlimited to, one or more phases of Ti--V--Zr--Ni material with C₁₄ andC₁₅ type crystal structures. Other Ti--V--Zr--Ni alloys may also be usedfor a rechargeable hydrogen storage negative electrode. One such familyof materials are those described in U.S. Pat. No. 4,728,586 ("the '586Patent") to Venkatesan, Reichman, and Fetcenko for Enhanced ChargeRetention Electrochemical Hydrogen Storage Alloys and an Enhanced ChargeRetention Electrochemical Cell, the disclosure of which is incorporatedby reference. The '586 Patent describes a specific sub-class of theseTi--V--Ni--Zr alloys comprising Ti, V, Zr, Ni, and a fifth component,Cr. The '586 patent, mentions the possibility of additives and modifiersbeyond the Ti, V, Zr, Ni, and Cr components of the alloys, and generallydiscusses specific additives and modifiers, the amounts and interactionsof these modifiers, and the particular benefits that could be expectedfrom them.

The V--Ti--Zr--Ni family of alloys described in the '586 Patent has aninherently higher discharge rate capability than previously describedalloys. This is the result of substantially higher surface areas at themetal/electrolyte interface for electrodes made from the V--Ti--Zr--Nimaterials. The surface roughness factor (total surface area divided bygeometric surface area) of the V--Ti--Zr--Ni is approximately 10,000.This value indicates a very high surface area. The validity of thisvalue is supported by the inherently high rate capability of thesematerials.

The characteristic surface roughness of the metal electrolyte interfaceis a result of the disordered nature of the material. Since all of theconstituent elements, as well as many alloys and phases of them, arepresent throughout the metal, they are also represented at the surfacesand at cracks which form in the metal/electrolyte interface. Thus, thecharacteristic surface roughness is descriptive of the interaction ofthe physical and chemical properties of the host metals as well as ofthe alloys and crystallographic phases of the alloys, in an alkalineenvironment. These microscopic chemical, physical, and crystallographicparameters of the individual phases within the hydrogen storage alloymaterial are believed to be important in determining its macroscopicelectrochemical characteristics.

In addition to the physical nature of its roughened surface, it has beenobserved that V--Ti--Zr--Ni metal hydride alloys tend to reach a steadystate surface condition and particle size. This steady state surfacecondition is characterized by a relatively high concentration ofmetallic nickel. These observations are consistent with a relativelyhigh rate of removal through precipitation of the oxides of titanium andzirconium from the surface and a much lower rate of nickelsolubilization. The resultant surface seems to have a higherconcentration of nickel than would be expected from the bulk compositionof the negative hydrogen storage electrode. Nickel in the metallic stateis electrically conductive and catalytic, imparting these properties tothe surface. As a result, the surface of the negative hydrogen storageelectrode is more catalytic and conductive than if the surface containeda higher concentration of insulating oxides.

The surface of the negative electrode, which has a conductive andcatalytic component--the metallic nickel--appears to interact withchromium alloys in catalyzing various hydride and dehydride reactionsteps. To a large extent, many electrode processes, including competingelectrode processes, are controlled by the presence of chromium in thehydrogen storage alloy material, as disclosed in the '586 Patent.

In contrast to the V--Ti--Zr--Ni based alloys described above, the earlyAB_(s) alloys are ordered materials that have a different chemistry andmicrostructure, and exhibit different electrochemical characteristicscompared to the V--Ti--Zr--Ni based alloys. However, recent analysisreveals while the early AB₅ alloys may have been ordered materials, morerecently developed AB₅ alloys are not. The performance of the earlyordered AB₅ materials was poor. However, as the degree of modificationincreased (that is as the number and amount of elemental modifiersincreased) the materials became disordered, and the performance of theAB₅ alloys began to improve significantly. This is due to the disordercontributed by the modifiers as well as their electrical and chemicalproperties. This evolution of AB₅ type alloys from a specific class of"ordered" materials to the current multicomponent, multiphase"disordered" alloys is shown in the following patents: (i) U.S. Pat. No.3,874,928; (ii) U.S. Pat. No. 4,214,043; (iii) U.S. Pat. No. 4,107,395;(iv) U.S. Pat. No. 4,107,405; (v) U.S. Pat. No. 4,112,199; (vi) U.S.Pat. No. 4,125,688; (vii) U.S. Pat. No. 4,214,043; (viii) U.S. Pat. No.4,216,274; (ix) U.S. Pat. No. 4,487,817; (x) U.S. Pat. No. 4,605,603;(xii) U.S. Pat. No. 4,696,873; and (xiii) U.S. Pat. No. 4,699,856.(These references are discussed extensively in U.S. Pat. No. 5,096,667and this discussion is specifically incorporated by reference.)

Simply stated, in the AB₅ alloys, like the V--Ti--Zr--Ni metal hydridealloys, as the degree of modification increases, the role of theinitially ordered base alloy is of minor importance compared to theproperties and disorder attributable to the particular modifiers. Inaddition, analysis of the current multiple component AB₅ alloysindicates that current AB₅ alloy systems are modified following theguidelines established for V--Ti--Zr--Ni based systems. Thus, highlymodified AB₅ alloys are identical to V--Ti--Zr--Ni based alloys in thatboth are disordered materials that are characterized bymultiple-components and multiple phases and there no longer exists anysignificant distinction between these two types of multicomponent,multiphase alloys.

Rechargeable alkaline cells can be either vented cells or sealed cells.During normal operation, a vented cell typically permits venting of gasto relieve excess pressure as part of the normal operating behavior. Incontrast, a sealed cell generally does not permit venting on a regularbasis. As a result of this difference, the vent assemblies and theamounts of electrolyte in the cell container relative to the electrodegeometry both differ significantly.

Vented cells operate in a "flooded condition." The term "floodedcondition" means that the electrodes are completely immersed in, coveredby, and wetted by the electrolyte. Thus, such cells are sometimesreferred to as "flooded cells." A vented cell is typically designed forvery low operating pressures of only a few pounds per square inch afterwhich excess pressures are relieved by a vent mechanism.

In contrast, sealed cells are designed to operate in a "starved"electrolyte configuration, that is with a minimum amount of electrolyteto permit gas recombination. The enclosure for a sealed cell is normallymetallic and the cell may be designed for operation at up toapproximately 100 p.s.i. absolute or higher. Because they are sealed,such cells do not require periodic maintenance.

Typically, a sealed rechargeable alkaline cell uses a cylindricalnickel-plated steel case as the negative terminal and the cell cover asthe positive terminal. An insulator separates the positive cover fromthe negative cell can. The electrodes are wound to form a compact "jellyroll" with the electrodes of opposite polarity isolated from each otherby a porous, woven or non-woven separator of nylon or polypropylene, forexample. A tab extend from each electrode to create a single currentpath through which current is distributed to the entire electrode areaduring charging and discharging. The tab on each electrode iselectrically connected to its respective terminal.

In sealed cells, the discharge capacity of a nickel based positiveelectrode is limited by the amount of electrolyte, the amount of activematerial, and the charging efficiencies. The charge capacities of a NiCdnegative electrode and a Ni--MH negative electrode are both provided inexcess, to maintain the optimum capacity and provide overchargeprotection.

The operational lifespan, that is, the available number of charge anddischarge cycles of a sealed cell, typically determines the kinds ofapplications for which a cell will be useful. Cells that are capable ofundergoing more cycles have more potential applications. Thus, longerlifespan cells are more desirable.

An additional goal in making any type of electrode is to obtain as highan energy density as possible. For small batteries, the volume of anickel hydroxide positive electrode is more important than weight andthe energy density is usually measured in mAh/cc, or an equivalent unit.

At present, sintered, foamed, or pasted nickel hydroxide positiveelectrodes are used in NiCd and Ni--MH cells. The process of makingsintered electrodes is well known in the art. Conventional sinteredelectrodes normally have an energy density of around 480-500 mAh/cc. Inorder to achieve significantly higher loading, the current trend hasbeen away from sintered positive electrodes and toward foamed and pastedelectrodes that can be manufactured with an energy density of greaterthan 550 mAh/cc.

In general, sintered positive electrodes are constructed by applying anickel powder slurry to a nickel-plated steel base followed by sinteringat high temperature. This process causes the individual particles ofnickel to weld at their points of contact resulting in a porous materialthat is approximately 80% open volume and 20% solid metal. This sinteredmaterial is then impregnated with active material by soaking it in anacidic solution of nickel nitrate, followed by conversion to nickelhydroxide by reaction with sodium hydroxide. After impregnation, thematerial is subjected to electrochemical formation in alkaline solutionto convert the nickel hydroxide to nickel oxyhydroxide.

In all rechargeable cells using a nickel hydroxide positive electrode,the nickel hydroxide changes back and forth between Ni(OH)₂ and NiOOH asthe cell is charged and discharged. These reactions involve asignificant density change during the charge/discharge reactions. Thisexpansion and contraction causes a "swelling" of the electrode. Thisswelling is a common cause of failure in cells using a nickel hydroxidepositive electrode. Failure occurs because as the positive electrodeswells, it absorbs free electrolyte from the separator until theseparator dries out.

U.S. Pat. No. 5,077,149, describes a cell system to avoid swelling ofthe positive electrode. The described cell uses a Ni--MH negativeelectrode, a nickel hydroxide positive electrode, and a sulfonated,non-woven polypropylene separator all of which contain a zinc compound.The zinc compound prevents electrolyte migration to the positiveelectrode by facilitating electrolyte retention in the negativeelectrode and the separator. This reduces the expansion of the positiveelectrode. This patent states that expansion of the positive electrodecauses a change in the electrolyte distribution and an increase ininternal resistance, and that the use of zinc oxide in the cell, ratherthan the fabrication of the electrode is the solution to this problem.

Various "poisons", introduced during the production of the positiveelectrode or generated during the operation of the cell, can also causecell failure. For example, residual nitrates and Fe are both knownpoisons.

Residual nitrates occur during impregnation processes that use nickelnitrate. Unfortunately, even parts per million levels of nitrate canresult in undesirable self-discharge mechanisms through the formation ofthe nitrate shuttle reaction.

In both NiCd and Ni--MH cells, free Fe can be leached frominsufficiently plated can or tab connections. In addition, some Ni--MHalloys contain Fe, and these materials oxidize and corrode. Once Fe getsinto the aqueous electrolyte solution, it is deposited on the nickelhydroxide and reduces the oxygen overvoltage, effectively, poisoning thepositive electrode. A reduction in the oxygen overvoltage means thatoxygen evolution will occur at the positive electrode before thepositive electrode is fully charged, resulting in a reduction incapacity. It has become standard practice in the Ni--Cd industry toavoid even the smallest Fe impurity in the cell by substituting pure Nifor Fe and by the extensive use of heavy nickel plating. In addition,previously unknown poisoning mechanisms such as deposition ordissolution of metallic species such as oxides of Ti, Zr, or V have beenshown to affect the nickel hydroxide electrode in adverse ways such asreduction in capacity, lowered cycle life, and increased self discharge.

In summary, prior art nickel hydroxide positive electrodes have a numberof deficiencies that prevent the realization of the full potential ofimproved Ni--MH negative electrodes. For example, sintered positiveelectrodes have energy density limitations. In addition, while the useof foamed and pasted electrodes avoid these energy density problems,presently available nickel hydroxide positive electrodes undergoswelling that ultimately results in separator dryout, are susceptible topoisoning, may have poor rate capability, and are susceptible topoisoning.

SUMMARY OF THE INVENTION

One object of the present invention is a sintered nickel hydroxidepositive electrode having an energy density of ≧560 mAh/cc.

Another object of the present invention is a nickel hydroxide positiveelectrode that is resistant to swelling.

Yet another object of the present invention is a nickel hydroxidepositive electrode that is resistant to poisoning.

These and other objects of the present invention are satisfied by apositive electrode for use in alkaline rechargeable electrochemicalcells comprising: a material comprising a compositionally andstructurally disordered multiphase nickel hydroxide host matrix whichincludes at least one modifier, preferable three modifiers, chosen fromthe group consisting of F, Li, Na, K, Mg, Ba, Ln, Se, Nd, Pr, Y, Co, Zn,Al, Cr, Mn, Fe, Cu, Zn, Sc, Sn, Sb, Te, Bi, Ru, and Pb.

Other objects of the present invention are satisfied by a positiveelectrode for use in rechargeable electrochemical cells comprising asintered nickel hydroxide electrode lacking cadmium and having an energydensity of ≧560 mAh/cc, and a cycle life of ≧500 cycles.

Still other objects of the invention are satisfied by a positiveelectrode for use in alkaline rechargeable electrochemical cells, saidpositive electrode lacking cadmium, having an energy density of ≧560mAh/cc, having a self discharge in a sealed Ni--MH cell of ≦30% in 30days at 20° C., and having residual nitrates present in an amount lessthan 200 ppm.

Additional objects of the invention are satisfied by a positiveelectrode for use with V--Ti--Zr--Ni metal hydride alloy rechargeableelectrochemical cells comprising a sintered nickel hydroxide electrodelacking cadmium and having an energy density of ≧560 mAh/cc, a cyclelife of ≧500 cycles, and a self discharge in a sealed Ni--MH cell of≦30% in 30 days at 20° C.

Objects of the invention are also satisfied by a sintered positiveelectrode for use in alkaline rechargeable electrochemical cellscomprising: a nickel substrate that is preoxidized and perforated; anickel sinter having pores and an outer surface on said nickelsubstrate; and nickel hydroxide and cobalt hydroxide precipitate in saidpores and on said outer surface; where said sintered positive electrodecontains <200 ppm residual nitrates, has an energy density of ≧560Ah/cc, has a self discharge in a sealed Ni--MH cell of ≦30% in 30 daysat 20° C., and lacks Cd.

Objects of the present invention are also satisfied by a process forfabricating sintered electrode material from which a sintered positiveelectrode for use in an alkaline rechargeable electrochemical cell canbe produced, said process comprising: forming a slurry of nickel powder,water, carboxy methyl cellulose binder, methyl cellulose binder, and apoly(ethylene oxide) polymer; spreading said slurry on a preoxidizedperforated nickel substrate; drying said slurry; and sintering saidslurry.

Yet other objects of the invention are satisfied by a process forimpregnating sintered electrode material from which a high loadinguniformly distributed multiphase substantially nitrate free sinteredpositive electrode for use in an alkaline rechargeable electrochemicalcell can be produced, said process comprising: impregnating saidsintered electrode material using from multiple impregnation cycles toattain high loading, where each impregnation cycle comprises the stepsof: placing said sintered electrode material on a rack; dipping saidrack into nickel nitrate; allowing said rack to drip dry; dipping saiddried rack into NaOH solution; spraying said rack in a first tank withdeionized water overflowing from a second tank; dipping said rack insaid second tank filled with deionized water overflowing from a thirdtank; dipping said rack in said third tank filling with deionized waterat a rate of 8-10 gpm; drying said rack; and flipping said rack toattain uniform deposition of material; where in the median dip cycle andin the final dip cycle of said multiple impregnation cycles, said stepof dipping said rack into nickel nitrate is replaced by a step ofdipping said rack into cobalt nitrate.

The objects of the present invention are also satisfied by a process forforming a high loading uniformly distributed multiphase substantiallynitrate free sintered positive electrode for use in an alkalinerechargeable electrochemical cell, said process comprising: (1)fabricating sintered electrode material by forming a slurry of nickelpowder, water, carboxy methyl cellulose binder, methyl cellulose binder,and a poly(ethylene oxide) polymer; spreading said slurry on apreoxidized perforated nickel substrate; drying said slurry; andsintering said slurry; (2) impregnating said sintered electrode materialusing multiple impregnation cycles to attain high loading, where eachimpregnation cycle comprises the steps of: placing said sinteredelectrode material on a rack; dipping said rack into nickel nitrate;allowing said rack to drip dry; dipping said dried rack into NaOHsolution; spraying said rack in a first tank with deionized wateroverflowing from a second tank; dipping said rack in said second tankfilled with deionized water overflowing from a third tank; dipping saidrack in said third tank filling with deionized water at a rate of 8-10gpm; drying said rack; and flipping said rack to attain uniform,deposition of material; where in the median dip cycle and in the finaldip cycle of said multiple impregnation cycles, said step of dippingsaid rack into nickel nitrate is replaced by a step of dipping said rackinto cobalt nitrate to produce an enriched cobalt surface; and (3)forming said impregnated sinter into positive electrode material bypresoaking said impregnated sinter in NaOH presoak tanks tosubstantially eliminate nitrates; brushing the presoaked impregnatedsinter in a surface brushing station; charging the brushed impregnatedsinter; discharging the charged impregnated sinter; rinsing thedischarged impregnated sinter; and drying said rinsed impregnated sinterto complete the formation of positive electrode material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional representation of the sintering line.

FIG. 2 is a schematic of the impregnation process.

FIG. 3 is a cross sectional representation of the deionized water rinsesystem.

FIG. 4 is a cross sectional representation of the formation line.

DETAILED DESCRIPTION OF THE INVENTION

A sintered positive electrode of the present invention embodies a newkind of active material, that has a energy capacity equivalent to foamedor pasted electrodes that have a porosity of 90% or greater, in additionthe active material of the present invention has a lower concentrationof residual nitrates, and a greater resistance to poisoning than priorart materials.

It is well known that Ni--MH negative electrode material is a much moreefficient storage medium than nickel cadmium negative electrodematerial. This makes it possible to decrease the thickness of thenegative electrode by 50% in a Ni--MH cell compared to a Ni--Cd cell andincrease the thickness of the positive electrode in a Ni--MH cell by50%. So, instead of two electrodes that are approximately 0.025" thickin a conventional C size Ni--Cd cell, Ni--MH cells have a 0.038" thickpositive electrode and a 0.012" thick negative electrode.

In order to make a thicker nickel hydroxide positive electrode; thepresent inventors discovered that the viscosity of the slurry mixtureused to prepare the nickel sinter structure must be much higher thanconventionally used in the Ni--Cd industry. A standard slurry forpreparing a nickel sinter structure for use in Ni--Cd cells has aviscosity of approximately 40,000 centipoise ("cp"). For a finalelectrode thickness of 0.025", the slurry is initially applied at athickness of approximately 0.05" followed by drying and sintering wherethe final thickness is reduced by approximately 50% or 0.025". After theslurry is applied to the substrate via a doctor blade apparatus, thecoated substrate is passed through a vertical drying tower, which isusually followed by a horizontal sintering operation.

To make a 50% thicker final electrode, the coating of the slurry ontothe substrate must be almost 0.090". This thickness is beyond thecapability of conventional slurries in that the normal viscosity is fartoo low to avoid sagging, running, and other imperfections as the coatedsubstrate enters a vertical drying tower. To correct this problem, thepresent invention involves a substantial increase in slurry viscosityfrom 40,000 cp to approximately 72-75,000 cp. This higher viscosityallows a much thicker coating of the slurry without the problemsdiscussed above.

In addition to making a substantially thicker electrode thanconventional nickel hydroxide electrodes, the present inventorsdiscovered that it is also desirable to make a nickel hydroxideelectrode that has higher capacity than previously known. Conventionalsintered nickel hydroxide electrodes have a base sinter structure thatis approximately 80% porous. Conventional thinking was that to make ahigher capacity electrode, it was necessary to make the nickel sintereven more porous, up to as high as 90%. Because 90% porous material thatis sintered is much more fragile and prone to swelling, the industrytrend has been to develop foamed and pasted structures of 90% porosity.

In fact, many companies have developed pasted nickel hydroxideelectrodes utilizing foam metal or fiber metal substrates having 90%porosity. Active material is usually applied by pasting techniques usingspecial active materials. The desired objective in using these pastedmaterials was primarily an increase in energy density from approximately500 mAh/cc for standard sintered structures to over 550 mAh/cc.

The trend toward foamed metal substrate electrodes away from sinteredelectrodes has advantages as well as disadvantages. The primarymotivating factors for foam metal substrate electrodes was a desire tohave a higher energy density than standard sintered electrodes, to lowerthe cost in high production volumes, and to simplify processing. On theother hand, a great number of problems exist which have still not beenfully addressed. The major problem with pasted electrodes that has notbeen resolved is that pasted electrodes generally have a low cycle lifeand poor power capability. The more porous structure is weak and veryprone to swelling resulting in electrolyte redistribution problems in asealed cell which shortens the cell's life. In addition, the higherporosity structure is also less conductive and the distance betweenactive material and current collection is usually increased. Bothproblems, swelling and reduced conductivity, have led to the developmentof very special active materials such as high density spherical nickelhydroxide. Intensive efforts are made to achieve high density in orderto achieve a high final electrode capacity. In addition, specialformulations of active material are utilized to reduce swelling.However, these electrodes are very costly due to the fact that the foammetal substrate and the high density nickel hydroxide are expensive toproduce.

Consequently, a sintered positive electrode having comparable energystorage capacity to pasted foam electrodes without reduced power andcycle life is very desirable.

Quite unexpectedly, the present inventors found that there was anabundant amount of active material present in a conventional nickelhydroxide positive electrode, but that it was not fully utilized. Informing a sintered electrode, nickel powder is sintered to form anelectrical and physical skeleton. Nickel hydroxide is deposited on thisskeleton in an impregnation process to form an electrically conductivematrix. The present inventors found that in the nickel powder skeleton,pores greater than approximately 30 microns in diameter could not fullyutilize the nickel hydroxide active material and that in conventionalnickel hydroxide positive electrodes, many of the pores were from 40-80microns in diameter. With this discovery, the present inventors realizedthat impregnation of more active material was not as important aseffectively utilizing the active material that was already present. Thesintered electrodes of the present invention predominantly limit poresize to approximately 30 microns in size, therefore, they have greatercapacity even though their overall porosity, like that of conventionalsintered electrodes, is around 80%.

One step in the construction of conventional sintered nickel hydroxideelectrodes involves forming a slurry of nickel and methyl cellulosebinder. The present inventors discovered that this simple, single binderslurry is not effective in providing good mixing and subsequent uniformpore size distribution. Methyl cellulose is a relatively inefficientbinder and, as a result, the nickel forms clumps and pockets. Theresulting sinter, if viewed under a microscope, shows the nickelskeleton to be uneven with many pores of greater than 30 microns insize. The inefficiency of the nickel/methyl cellulose slurry isexacerbated when trying to make a 0.038" thick positive electrode asdesired in a Ni--MH cell. In order to get a final thickness of 0.038",it is necessary to doctor blade the nickel slurry onto the substrate ina layer that is approximately 0.090" thick. This is approximately doublethe thickness of nickel slurry used to produce a conventional nickelhydroxide positive electrode. As previously stated, in order to achievesuch a thick electrode, it is necessary to substantially increase theslurry viscosity. The higher viscosity makes the mixing properties ofthe methyl cellulose even worse. Applying such a thick slurry isextremely difficult because conventionally formulated slurries lacksufficient viscosity to be self-sustaining at this thickness.

The problems associated with prior art slurries are overcome by a uniquebinder system we have developed. This binder system is formulated bycombining a poly(ethylene oxide) polymer, carboxymethyl cellulose, andmethyl cellulose with water. This slurry system is preferably formulatedusing 43-53 wt % nickel, 45-55 wt % water, 0.3-1.3 wt % POLYOX (a trademark of Union Carbide for poly(ethylene oxide) polymer), 0.1-0.9 wt %carboxymethyl cellulose, and 0.1-0.7 wt % methyl cellulose. Withoutwishing to be bound by theory, it is believed that this binder systemmakes it possible to produce higher capacity positive electrodes,because it mediates the even mixing and formation of a homogeneoussolution of nickel particles in a slurry that is extremely viscous yetcan still be doctor bladed into a uniform layer.

The slurry of the present invention is formulated by first mixing thedry ingredients, the poly(ethylene oxide) polymer, carboxymethylcellulose, and methyl cellulose with INCO 255° nickel powder(manufactured by the International Nickel Company) for 15 to 60,preferably 30 minutes by tumbling at from 1 to 3, preferably 2 RPM, in asmooth wall container. Water is added to the tumbled mixture and thetumbling resumed at from 1 to 3, preferably 2 RPM until the slurry has aviscosity of from 68,000 Centipoise (cp) to 76,000 cp; and a density offrom 1.65 to 1.71 g/cc.

The prepared slurry is doctor bladed onto a preoxidized perforatednickel substrate, followed by drying and sintering. Preoxidizedperforated nickel substrate is preferred for two reasons. It has beenfound that adhesion of the nickel powder is enhanced during sintering bypreoxidation. This is important in that delamination of the nickelpowder matrix is a common difficulty in the preparation of thesematerials. It has also been found that the preoxidized material is moreimmune to corrosion or "acid attack" during impregnation. It is alsoenvisioned that a full range of porous, solid nickel substrates may beused in addition to the perforated nickel substrate specificallydescribed herein.

FIG. 1 illustrates a roll-to-roll machine for doctor blading the slurryof the invention onto a substrate, drying the slurry, and sintering theslurry as a continuous operation. In FIG. 1, preoxidized perforatedsolid nickel substrate is fed from a payout roll 1 through a slurry box2. Previously prepared slurry is fed from the slurry barrel 3 into theslurry box 2. The thickness of the slurry on the substrate is determinedas the screen moves by the doctor blade 4. The slurry is dried in thevertical drying oven 5 and sized using a calendar roll 6. The sized dryslurry is sintered in a continuous sintering furnace 8, which can havemultiple heating zones and cooling zones. Samples for quality controlinspection are taken from the resulting sinter formed on the solidnickel substrate in an on line punch press device 9. Tension on thecontinuous ribbon of sinter as it moves through the machine ismaintained through the use of proximity switches such as proximityswitch 7 which can be connected through a control device such as a microcomputer (not shown) to the payout roll 1 and the take up roll 10.

Other unique aspects of the sintering process are the use of a specialdoctor blade apparatus which substantially eliminates the formation of"dogbone" shaped edges. It is common for the slurry to be wiped away instrips which can later be utilized as integral tabs for currentcollection. During the wiping, tremendous hydrostatic pressures arebuilt up causing a spring back effect of the slurry at the outer edgesnear the wiped areas. The doctor blade apparatus reduces this problemthrough the use of a tapered or stepped doctoring blade. The problem of"dogboning" is undesirable in that uneven compression can result in thecell unless expensive milling is done, essentially grinding away ofmaterial. Another unique solution to this problem is the use of acalendering mill positioned after drying, but before sintering. Thisallows the "dogbone" to be gently squeezed whereas post sintercalendering can cause a high density of nickel at the surface,inhibiting the penetration of nickel nitrate during impregnation.Finally, it should be noted that an atmosphere of 7% H₂ with the balanceof N₂ at a temperature of approximately 900° C. is used duringsintering. This atmosphere is most effective in providing a reducingatmosphere and high strength compared to other gas compositions.

Chemical conversion as a method of impregnating nickel hydroxide into asinter is well known. However, the chemical conversion of the presentinvention is unique and not suggested by the prior art. At least oneprior art method of impregnating the sinter also had otherdisadvantages. This method involves dipping the sintered nickel skeletoninto the acidic nickel nitrate solution which resulted in tremendous ofcorrosion of the nickel sinter.

Prior art methods of impregnation were also inappropriate for thepurpose of providing high loading of active material in order to achievethe desired high energy density. Frequently, problems of surface loadingprevented interior loading of active material, thereby inhibiting thegoal of thoroughly loading the available porosity from sintering withactive material. Other problems with prior art chemical impregnationinvolve insufficient drying after nitrate loading and rinsing, whichprevents high density nickel hydroxide loading. Still other problemsinvolved too high a level of residual nitrates and carbonate impuritiesdue to insufficient rinsing and purity of the NaOH conversion bath.Other problems with prior art impregnation involved the use of excessivetemperatures during drying, causing the creation of electrochemicallyinactive nickel oxides. Further problems resulted from a failure tocompensate for excessive loading variances due to gravity.

Finally, the active material itself is insufficient for use in Ni--MHcells. This problem is in addition to difficulties in producing a thick,highly loaded, high utilization electrode free from excessiveimpurities, discussed above. Prior art active materials did not have toaddress the problems of electrochemical formation difficulties due tohigh loading, poisoning resistance to new unique potential poisons froma V--Ti--Zr--Ni--Fe based alloys, nor the higher sensitivity of Ni--MHcells to self-discharge compared to NiCd cells. Further, electrodecharge efficiency, --especially under quick charge conditions had toimprove. This is in part due to the fact that practical currentdensities increase in high energy Ni--MH cells compared to NiCd cellsbecause end users still require ten hours of slow charging and 15 to 60minutes of fast charging, even though the absolute current required toone-hour charge a 5.0 Ah Ni--MH C size cell is 5.0 Amps compared to aNi--Cd C size cell where the one hour charge current is only 2-2.4 Amps.

In addition to the difficulties discussed above, problems also arise inthe electrochemical formation process. We have observed thatconventional prior art formation is totally inadequate in the areas ofresidual nitrate reduction and activation of the nickel hydroxide toeliminate discharge reserve. Further, we have also learned that thecomposition of the alkaline electrolyte used in formation, itstemperature, the charge and discharge current densities, and the mannerof rinsing and drying of the electrode all play important roles in theproper functioning of positive electrodes used in a high performanceNi--MH battery.

Together, these factors illustrate that the sintered nickel hydroxidepositive electrodes of the present invention are superior for use inNiCd cells and particularly superior for use in state of the art Ni--MHcells. The inadequacy of prior art sintered nickel hydroxide positiveelectrodes is underscored by the industry wide movement to foam basedpasted positive electrodes in Ni--MH cells as discussed above.

The present invention avoids the problems of the prior art. Rather thanusing nickel plated steel as the substrate, the present invention uses apreoxidized perforated nickel substrate, as discussed above. Thispromotes better adhesion of the nickel powder particle to the substratemetal as well as significantly limiting the amount of corrosion thatoccurs during the impregnation steps. In addition, the sintering processof the invention provides an additional degree of pre-oxidation to thenickel powder particles making them more resistant to corrosion duringimpregnation.

The impregnation process of the present invention is accomplished usingmultiple, successive, impregnation cycles.

A further aspect of the unique impregnation process of the presentinvention is that the positive electrode of the present inventioncontains a higher percentage of co-precipitated cobalt than do the priorart materials. While the use of co-precipitated cobalt is known,particularly for nickel-cadmium cells, the Co content of these cells isonly approximately 1-3%. in contrast, a positive electrode of thepresent invention contains greater than 6 wt %, preferably 9-10 wt %co-precipitated cobalt.

In the impregnation process of the present invention, the high cobaltcontent of the positive electrode is further accentuated by the use of acobalt nitrate dip in the median dip cycle and then again at the finaldip cycle. Herein, the phrase "median dip cycle" is used to refer to thedip cycle halfway through the series of successive impregnation cyclesand the phrase "final dip cycle" to refer to the dip cycle at the end ofthe series of successive impregnation cycles. Using the cobalt nitratemedian and final dip cycles produces an enriched cobalt surface whichprovides better conductivity, poisoning resistance, and suppresses O₂evolution.

Without wishing to be bound by theory, it is believed that the finalcobalt nitrate dip in the impregnation process also result in surfaceenriched cobalt. The median dip cycle is believed to improve utilizationand accelerate activation by increasing the overall conductivity of theaggregate active material. The outer surface is critical for poisoningresistance. In reality this means that while the composition of cobaltin the active material is greater than 6 wt %, preferably 9-10 wt %, theconcentration of cobalt hydroxide on the surface of the electrode ismuch higher.

Cobalt hydroxide is more resistant to poisoning than pure nickelhydroxide. The use of the described concentration of pure cobalthydroxide and a higher concentration of the co-precipitated cobalt iscrucial for use in V--Ti--Zr--Ni metal hydride alloy based Ni--MHbatteries. Another unique aspect of the materials of the presentinvention believed to result from this unique impregnation and formationprocess is that the finished electrode has a very low concentration ofresidual nitrates. Residual nitrates result primarily because theimpregnation cycles of the impregnation process use nickel nitrate orcobalt nitrate that are converted into their respective hydroxides asdescribed in detail below. In the prior art, the conversion of nitratesinto hydroxides was much less than 100% efficient and, as a result, somenickel nitrate was locked into the matrix of the finished positiveelectrode materials. In the present invention, the impregnation processis significantly more efficient which results in a reduction of theresidual nitrate to no more than 200 ppm, maximum.

The impregnation process of the present invention is schematicallyillustrated in FIG. 2. The sinter is coiled onto a rack and dipped intonickel nitrate or cobalt nitrate depending on the impregnation cycle.During the first impregnation cycle, a nickel nitrate dip, the sinter isdipped into 0.02N HO₃ in nickel nitrate (2.5M Ni(NO₃)₂) for 15 minutes.During the remainder of the impregnation cycles that use 0.02N HO₃ innickel nitrate the sinter is dipped in 0.04N HO₃ in nickel nitrate (2.5MNi(NO₃)₂) for 15 minutes. The nickel nitrate solutions are maintained atapproximately 45° C. The reduced acid concentration on the first cycleacts to inhibit corrosion especially at the substrate/sinter interface.On subsequent dips, corrosion is minimized by the nickel hydroxidereaction product itself. Each of the two cobalt nitrate impregnationcycles are for 15 minutes. The cobalt nitrate solutions are maintainedat approximately 20° C.

The dipped sinter is then dried until "bone dry". As used herein, "bonedry" means drying until no further weight loss occurs. The dipped sinteris usually bone dry after approximately 60 minutes at a temperature nothigher than 80° F. It has been learned that higher drying temperatureswere detrimental due to the formation of electrochemically inactivenickel oxide. Drying time is maintained at commercially practical levelby an emphasis on the circulation of large air volume as opposed tohigher temperature.

The bone dry sinter is then dipped in a 70° C., 30 wt %, solution ofNaOH for approximately 15 minutes, preferable 19-21 minutes, mostpreferably 20 minutes.

Following the NaOH dip, the sinter is rinsed using a three stepdeionized water rinse system as schematically diagrammed in FIG. 3,employing a spray tank 1, and two dip tanks 2, 3. The water in the spraytank 1 is the cascade overflow from dip tank 2, and the water in diptank 2 is the cascade overflow from dip tank 3. The water in the spraytank 1 is drained off at a rate of 8-10 gpm, the same rate at which thedeionized water is replenished to dip tank 3. The water in the spraytank 1 is sprayed from the top and bottom at the rate of approximately100 gpm. The term "deionized water" as used herein refers to waterhaving a maximum conductivity of 0.83 micro ohms, a minimum resistivityof 12,000 megaohms-cm, and a pH of from 4.5-7.5. The temperature of thewater as it is added to dip tank 3 is approximately 70°-80° C. A sinteris rinsed in each rinsing station for approximately 60 minutes. We havediscovered that a combination of spray/immersion is superior to either atotal spray or total immersion system. The system of the presentinvention exploits the better quality rinse resulting from immersion andthe efficiency of a spray to remove the initial NaOH quickly. After thecompletion of rinsing the sinter is again dried until bone dry.Impregnation cycles are repeated until the loading process is complete.

As previously mentioned, it is common in industry for the bottommaterial to be more heavily loaded than the top due to gravity. This isundesirable since loading is directly proportional to cell capacity anda low as possible capacity distribution is desirable. Some companiescombat this problem by restacking racks at certain points in the dipprocess. An aspect of the present invention involves a far superiorsolution. In the present invention, a rack flip device is used to flipeach impregnation rack so that the top of the rack becomes the bottomand the bottom becomes the top. This rotation takes only seconds, andthe rack/flip device is easily incorporated into the processing line.Using the rack flip device after each impregnation cycle insures uniformloading which is vital for achieving a uniformly high capacityelectrode.

Nitrate ions, as mentioned above, are generated in the electrodefabrication process of the present invention because of the use ofnickel and cobalt nitrates. When a sinter is immersed in a nickel orcobalt nitrate solution it soaks up the solution into its pores. Thesubsequent drying step drives off the water and leaves behind nickelnitrate or cobalt nitrate salt in the pores. When the dry sinter isimmersed in sodium hydroxide solution precipitation occurs. For nickelnitrate, for example, this precipitation can be expressed as follows:

    Ni(NO.sub.3).sub.2 +2NaOH→Ni(OH).sub.2 +2NaNO.sub.3

The nickel or cobalt hydroxide precipitate is held in the pores and onthe surface of the nickel sinter and the sodium nitrate dissolves in thehydroxide. Some of the nitrate can remain occluded in the nickelhydroxide or cobalt hydroxide precipitate.

In prior art methods, the impregnated sinter inevitably retains smallquantities of nitrate in spite of any subsequent rinsing operation. Wehave determined that the actual amount of nitrate held depends on thenumber of dips (the loading) and the amount of residual nitrates in thedip tanks and rinse water. In the present invention, the three partrinse following the NaOH dip removes nitrate residue and gives an activematerial filled only with nickel/cobalt hydroxides. However, it is nownecessary to follow the rise of sodium nitrate in the alkali dipsolution and also in the rinse water. Prior art methods that did not useas many impregnation cycles and/or the three part dip described abovepermit the concentration of nitrates to build up to levels sufficient toeffect self discharge. Consequently, an aspect of the present inventionmonitors the nitrate ion level in the NaOH bath and when the levelreaches a maximum of 30,000 ppm, the solution is replaced with freshNaOH.

In addition, the formation process of the present invention removesnitrates that remain in the impregnated sinter. In addition, theformation process increases the surface area of the electrode andincreases the electrolyte uptake for quicker activation. The formationprocess occurs after the nickel hydroxide has been deposited inside thepores in the sintering step and involves an electrochemical formationcycle that is a one cycle charge/discharge prior to the positiveelectrode material going into the battery. After the completion of theformation process, the amount of residual nitrates present is small,despite the fact the electrode is approximately 50% thicker and 10-15%heavier due to loaded active material, both factors that wouldcontribute to higher levels of residual nitrate rather than lowerlevels. The effectiveness of the formation process is a result of highefficiency through 200% overcharge followed by complete discharge.

FIG. 4 schematically illustrates the formation process of the presentinvention in which the impregnated sinter is formed into sinteredpositive electrode material. The formation process begins by windingimpregnated sinter from the impregnation racks onto formation spools.

A formation spool 1 is fed into a presoak tank 2 containing NaOHelectrolyte at 40°-50° C., preferably 45° C. We have found that theelectrolyte absorbed by the electrode during the presoak greatlyfacilitates charge efficiency during the initial charging step. Theelectrode material is unwound from the formation spool 1 and fed pastopposing brushes 3 to remove surface loading and loose particulates. Thebrushes have variable speed and pressure to allow adjustment forspecific incoming material and conditions. The brushing step furtherfacilitates electrolyte uptake by removing surface loading. In addition,brushing improves electrical contact between the material and contactrollers because it removes surface nickel hydroxide which has lowconductivity.

Using a series of wetted contact rollers, the material is moved througha charge section 3. The charge section 3 consists of a series of tanks,preferably four tanks, containing electrolyte maintained at the sametemperature as the presoak tank 2. A counter electrode is present ineach tank of the charge section 3. The counter electrode is connected tothe negative terminal of a power supply and the wetted contact rollersare connected to the positive terminal of the power supply.

The charge section is designed to provide 200% of the theoreticalcapacity of the material. However, no amount of charge input will beeffective if it is not accepted by the active material. We have inventedseveral innovative approaches to assure charge acceptance. As mentionedabove, the use of a presoak tank and brushes improves electrolytepenetration. However, the solution itself is also important. It iscommon in industry to form in KOH rather than NaOH since KOH is used inthe final cell for reasons of charge efficiency, temperaturecharacteristics, cycle life, etc. Consequently, our discovery that NaOHis a more effective formation electrolyte is surprising. A relatedaspect of the present invention is the discovery that heating the NaOHto 45° C. rather than conventional formation at room temperature furtheraccentuates the benefit. We have observed substantially higher capacityon the first cycle in the sealed cell when using NaOH at 45° C. to thedegree that almost 100% of expected capacity is provided on the firstcycle.

Still another aspect of the invention is a means to provide greatercharge acceptance by an innovative counter electrode design whichcompensates for voltage drop across the electrode material. Normalformation uses a single flat plate counter electrode with terminalconnections at the top of the bath. This is a problem in that theterminal connection of the material itself is the contact roller, whichis also at the top to the bath. The resistance of the nickel electrodeis substantial and over the entire length of the counter electrode, thevoltage drop is significant. A conventional single plate counterelectrode causes significant variances in the current density from thetop to the bottom of the material. This situation results in most of theapplied current being wasted on gas evolution instead of being used tocharge the material. We confirmed this conclusion using static tests onthe machine that showed the characteristic color change from green(nickel hydroxide) to black (nickel oxyhydroxide) occurred only in theupper 10% of the available charge section.

Our innovative solution to this problem was to break the single platecounter electrode into five segments, where each segment is separated bya resistor designed to match the voltage drop of the positive electrodematerial. This configuration provides a very uniform current density tothe material, facilitating charge. This same approach facilitatesdischarge as well, and is repeated throughout the formation process.

The collective formation process provides electrodes having virtually a100% real depth of discharge, greatly reduced levels of residualnitrates, increased surface area (which allows easy electrolyte uptakeduring cell fabrication as mentioned above), and yields cells thatexhibit virtually 100% of their expected capacity even on their firstcycle. As a further demonstration of the effectiveness of the describedformation process, conventional Ni--Cd positive electrodes not sinteredor impregnated in the manner described above yield cells having veryhigh self-discharge. By making no change in the fabrication of suchelectrodes except to use the improved formation described above,self-discharge rates were reduced by 50%. Thus, the cells of the presentinvention have a self discharge in a sealed Ni--MH cell of ≦30% in 30days at 20° C.

The discharge section 4 of FIG. 4 is similar to the charge section 3.The discharge section 4, also consists of a series of tanks containingelectrolyte at the same temperature as the presoak tank 2, counterelectrodes, and wetted contact rollers. However, in the dischargesection 4, it is necessary to have only two tanks because discharge isaccepted at higher rates than charge, and only 100% of capacity isrequired to be removed, (unlike charge where 200% input is required andcharging alone has "activated" the material). The counter electrodes areconnected to the positive terminal of a power supply, and the wettedcontact rollers are connected to the negative terminal of the powersupply. The object of the discharge section 4 is to remove all of thecharge provided in the charge section 3. Since not all electrodematerial has identical capacity, the discharge section is designed toprovide approximately 6% overdischarge (on average) in order to ensurethat all material is fully discharged. Generally, nominally overchargedmaterial has a characteristic "grayish" color.

The rinse 5 of the formation process preferably uses three tanks ofdeionized water having a countercurrent flow rate of 3-5 gpm and aninitial temperature of 75°-85° C. Brushes 6 may optionally be present inat least one of the rinse tanks to remove surface loading andparticulate matter.

The dryer 7 must be capable of drying the material until it is bone dry.Any kind of appropriate dryer may be used such as an infrared dryer.

Finished positive electrode material is taken up on the take up spool 8.

The multiple impregnation cycles of the present invention result inspatial and orientational placement of similar or dissimilar atoms orgroups of atoms that produce qualitatively new performance levels forsintered positive electrodes. The multiple impregnation cycles of thepresent invention result in a disordered multicomponent materialcomprising a nickel hydroxide host matrix into which cobalt isincorporated as a modifier in a manner similar to the negative alloydisordered materials described above. The disordered positive alloymaterials of the present invention do not have periodic local order.

By forming nickel hydroxide positive electrodes that are disorderedmaterials, we have greatly increased the porosity and performance ofthese electrodes. Generally, the improved characteristics of thesealloys result from tailoring the local chemical order and hence thelocal structural order by the incorporation of at least one modifier,most preferably three modifiers, chosen from the group consisting of F,Li, Na, K, Mg, Ba, Ln, Se, Nd, Pr, Y, Co, Zn, Al, Cr, Mn, Fe, Cu, Zn,Sc, Sn, Sb, Te, Bi, Ru, and Pb. Like the metal hydride negative alloysdiscussed above, disordered positive electrode materials have asubstantially increased density of catalytically active sites andstorage sites compared to the prior art single or multi-phasecrystalline materials. These additional sites are responsible forimproved efficiency of electrochemical charging/discharging and anincrease in electrical energy storage capacity.

The choice of disordered materials has fundamental scientificadvantages: as seen, a substantial number of elements can be included inthe list of candidates for electrodes. These elements offer a variety ofbonding possibilities due to the multi-directionality of d-orbitals, andless so due to f-orbitals which, although extending in still moredirections than d-orbitals, are closer to the nucleus of the metal atomand, hence, less accessible.

Where prior art sintered electrode materials had an energy density ofonly around 500 mAh/cc, the materials of the present invention have anenergy density of ≧560 mAh/cc, preferably 600 mAh/cc.

The present invention is explained further in the following non-limitingExamples.

EXAMPLES

                  TABLE 1                                                         ______________________________________                                        SLURRY FORMULATION                                                                            quantity in kg                                                ______________________________________                                        Nickel Powder     54.9                                                        Water             58.5                                                        carboxy methyl cellulose                                                                        .626                                                        methyl cellulose  .478                                                        Polyox ®      .956                                                        total             115.46                                                      ______________________________________                                    

A slurry was prepared using nickel powder, water, carboxy methylcellulose binder, methyl cellulose binder, and POLYOX® poly(ethyleneoxide) polymer in the quantities indicated in Table 1.

All the materials except water were added to a mixing drum which wasrotated for thirty minutes at 2 rpm. Water was then added and mixed inwith a stirring rod to remove air and reduce lumps to less than 0.5inches in diameter. The drum was again sealed and mixed at 2 rpm. After48 hours, the density of the resulting slurry was checked and anyvisible lumps broken. Viscosity was adjusted to 72,000 Centipoise (cp)(+/-4,000 cp) and density to 1.68 g/cc (+/-0.03 g/cc).

Sintering took place in a five zone furnace with each zone set atapproximately 910° C. Prior to sintering, the slurry was doctor bladedonto a preoxidized perforated solid nickel substrate and dried in a twozone drying tower at temperatures of 107° C. and 88° C. The air flow wasmaintained at 5 SCFM. Drying took place under a 7% hydrogen, 93%nitrogen atmosphere. The physical parameters of these materials at thispoint are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                  after     after    after                                                      drying tower                                                                            sintering                                                                              doctor blade                                     ______________________________________                                        thickness (in)                                                                            0.094       0.045    0.036                                        area weight (g/in.sup.2)                                                                  2.21        1.241    1.18                                         Density (g/in.sup.3)                                                                      23.5        27.6     32.6                                         ______________________________________                                    

Sintered material was then coiled on impregnation racks. Theimpregnation process involved 14 individual impregnation cycles. Eachimpregnation cycle involved a nitrate dip in nickel or cobalt nitrate,drying, a alkaline dip in NaOH, rinsing, and drying.

The nitrate dip of impregnation cycle 1 was in 0.02N HO₃ in 2.5MNi(NO₃)₂ hexahydrate for 20 minutes. The nitrate dips for impregnationcycles 6 and 14 were in cobalt nitrate hexahydrate for 20 minutes. Thenitrate dips for all other impregnation cycles used 0.4N HO₃ in 2.5MNi(NO₃)₂ hexahydrate for 15 minutes.

For each impregnation cycle, following the nitrate dip in theappropriate nitrate solution, the impregnation rack was lifted out ofthe solution and allowed to drip dry for 30 minutes. The rack was thenplaced in a forced air recirculation dryer at 80° C. for 60 minuteswhere the dryer had a flow rate of 2,000 ft² /minute.

Rinsing following the nitrate dip, was done in a three tank systemhaving a counter current flow from tank 3 to 2 to 1. Tank 1 was a sprayrinse and tanks 2 and 3 were immersion rinses. Deionized water was usedthroughout.

Following the final impregnation cycle, impregnated positive materialwas uncoiled from the impregnation racks and coiled onto formationspools. The material was then fed continuously at a rate of 8"/minuteinto the formation machine. In the formation machine the formationspools were placed in a presoak tank containing 30% NaOH electrolyte at45° C. The material was then fed from the formation spools throughopposing nylon brushes (to remove surface loading and looseparticulates), and into a charge section.

The charge section consisted of four tanks containing counter electrodesand contact rollers. In the charge section, the contact rollers wereconnected to the positive terminal of a power supply while counterelectrodes were connected to the negative terminal. This sectionprovided the electrode material with at least 90% of its theoreticalstate of charge in order to encourage electrolyte absorption and cellcapacity, as well as to remove electrochemically inactive chargereserve. Electrolyte in the charge section was maintained at atemperature of 45° C. to assist the charging reaction efficiency. Also,the electrolyte was recirculated and sprayed onto the contact rollersand tension on the belt of the material was kept high in order toprovide optimal conductivity between the rollers and the material.

The material was then passed into a discharge section consisting of twotanks where it received approximately a 6% overdischarge, on an average,in order to insure that all material was fully discharged.

The material was then rinsed using deionized water and nylon brushes.

Finally, the material was dried using an infrared heater. The resultingpositive electrode belt was slit, punched, cut to length, and fabricatedinto standard positive electrodes.

Ni--MH negative electrode material having the following composition

    V.sub.18 Ti.sub.15 Zr.sub.18 Ni.sub.29 Cr.sub.5 Co.sub.7 Mn.sub.8

was fabricated into negative electrodes as described in copending U.S.application Ser. No. 07/879,823 the contents of which are incorporatedby reference.

Standard nickel cadmium negative electrode materials were fabricated asdescribed in Falk and Salkind, Alkaline Storage Batteries (1969).

Prepared negative electrodes, separator, nickel hydroxide positiveelectrodes of the present invention, and 30% KOH electrolyte wereassembled into "C" cells as described in detail in U.S. patentapplication Ser. No. 07/879,823. The specific negative electrode,separator, and positive electrode used in each cell is indicated inTable 3, below. The finished cells were subjected to charging anddischarging conditions and their charge retention determined asindicated in Table 3.

                  TABLE 3                                                         ______________________________________                                                             charge retention                                         positive negative              @C rate to 1.0 V                               alloy    alloy      separator  3 days                                                                              14 days                                  ______________________________________                                        I-pos    Ni-MH      treated pp 92    79                                       I-pos    Ni-MH      pp         83    62                                       I-pos    Ni-MH      nylon      81    52                                       S-pos    Ni-MH      treated pp 88    67                                       S-pos    Ni-MH      pp         78    42                                       S-pos    Ni-MH      nylon      76    39                                       ______________________________________                                         "NiMH" stands for a NiMH alloy having the composition V.sub.18 Ti.sub.15      Zr.sub.18 Ni.sub.29 Cr.sub.5 Co.sub.7 Mn.sub.8 ;                              "Ipos" electrodes are positive electrodes fabricated according to the         present invention;                                                            "Spos" electrodes are standard, prior art, positive electrodes;               "nylon" separators are standard nylon separators;                             "pp" separators are standard untreated polypropylene separators; and          "treated pp" separators are radiation grafted polypropylene separators as     described in detail in U.S. Pat. application No. 07/879,823.             

It is obvious to those skilled in the art that the positive electrodematerials of the present invention may be prepared by additional methodswithout departing from spirit and scope of the present invention.

The drawings, discussion, descriptions, and examples of thisspecification are merely illustrative of particular embodiments of theinvention and are not meant as limitations upon its practice. It is thefollowing claims, including all equivalents, that define the scope ofthe invention.

What is claimed is:
 1. A positive electrode for use in alkalinerechargeable electrochemical cells comprising:a material comprising acompositionally and structurally disordered multiphase nickel hydroxidehost matrix which includes at least one modifier chosen from the groupconsisting of F, Li, Na, K, Mg, Ba, Ln, Se, Nd, Pr, Y, Co, Zn, Al, Cr,Mn, Fe, Cu, Zn, Sc, Sn, Sb, Te, Bi, Ru, and Pb.
 2. The positiveelectrode as claimed in claim 1, comprising at least three modifierschosen from the group consisting of F, Li, Na, K, Mg, Ba, Ln, Se, Nd,Pr, Y, Co, Zn, Al, Cr, Mn, Fe, Cu, Zn, Sc, Sn, Sb, Te, Bi, Ru, and Pb.3. The positive electrode as claimed in claim 1 for use withV--Ti--Zr--Ni metal hydride alloy rechargeable electrochemical cells. 4.The positive electrode as claimed in claim 1 for use with V--Ti--Zr--Nimetal hydride alloy rechargeable electrochemical cells wherein saidpositive electrode further comprises a sintered nickel hydroxideelectrode lacking cadmium and having a cycle life of ≧500 cycles.
 5. Thepositive electrode as claimed in claim 1 for use with V--Ti--Zr--Nimetal hydride alloy rechargeable electrochemical cells wherein saidpositive electrode further comprises a sintered nickel hydroxideelectrode lacking cadmium and havingan energy density of ≧560 mAh/cc, acycle life of ≧500 cycles, and a self discharge in a sealed Ni--MH cellof ≦30% in 30 days at 20° C.
 6. The positive electrode claimed in claim5, wherein residual nitrates are present in an amount less than 200 ppm.7. The positive electrode claimed in claim 5, wherein said energydensity is greater than 600 mAh/cc.
 8. The positive electrode claimed inclaim 5, comprising a sintered electrode material having poresapproximately 30 microns in size.
 9. The positive electrode claimed inclaim 8, wherein the overall porosity of said sintered electrodematerial is around 80%.
 10. The positive electrode claimed in claim 5,wherein said positive electrode contains greater than 6 wt %coprecipitated cobalt.
 11. The positive electrode claimed in claim 5,wherein said positive electrode contains 9-10 wt % coprecipitatedcobalt.